The present invention relates to fluidized bed systems and, more particularly, to a method of feeding particulate material to fluidized furnace.
In a typical present day fluidized bed furnace, particulate fuel, such as coal having a top size ranging from up to about 6.5 mm., is typically fed to and combusted within a fluidized bed of similar size particulate material at relatively low temperatures ranging from 760.degree. C. to 925.degree. C. Fluidized bed furnaces are particulately adaptable to burning sulfur containing fuels as the particulate material making up the bed may include a sulfur absorbent, most commonly crushed limestone, in addition to the particulate fuel. Fluidizing air, which also serves as combustion air, is supplied to the fluidized bed from a air plenum located beneath the bed support plate. The fluidizing air passes upwardly from the air plenum into the fluidized bed through a plurality of openings in the support plate at a flow rate sufficiently high to fluidize or entrain the particulate material within the fluidized bed depending upon the velocity of the fluidizing air. In a typical bubbling bed system, the velocity of the fluidizing air is controlled to fluidize, but not entrain, a majority of the particulate material in the bed. In a typical circulating fluidized bed, however, the velocity of the fluidizing air is maintained high enough to entrain most of the particulate material.
A number of different approaches have been suggested for feeding particulate material to the bed, including overbed feed systems and underbed feed systems. In typical fluidized bed furnace feed systems, whether of the overbed feed type or the underbed feed type, or a combination thereof, separate feed systems are utilized for each of the different particulate materials being supplied to the bed. Typically, limestone is crushed in the yard and transported to the boiler house where it is stored in storage silos for subsequent feeding to the fluidized bed within the furnace through either an overbed or underbed feed system. In a bubbling bed furnace, the portion of particulate material elutriated from the fluidized bed and entrained in the fluidizing gas is removed from the fluidizing gas in a mechanical collector and transported to storage tanks for recycle back to the fluidized bed. Typically this recycle material will contain partially combusted coal, commonly referred to as char, unreacted sulfur oxide absorbent, sulfate salts formed on the reaction of the absorbent with sulfur dioxides in the flue gas, and fly ash particles generated upon the combustion of the coal.
On bubbling bed furnaces, an additional feed system is typically supplied for drying and transporting crushed coal to the fluidized bed of the furnace. The coal is pneumatically transported in a stream of hot recycled flue gas from the coal crushers to a pair of cyclone separators. As the coal is transported, a major portion of the moisture associated with the coal is evaporated by flash drying in the presence of the hot flue gas. The coal is separated from the flue gas in the cyclone separators and transported on conveyers to a storage silo for subsequent feeding directly to the fluidized bed. The flue gas, together with any fine coal particles entrained therein, is vented from the cyclone separators to a bag filter wherein the fine coal particles are removed from the flue gas before the relatively clean flue gas is vented to the atmosphere. The coal fines recovered in the bag filters are conveyed to the coal storage silos for subsequent feeding to the furnace.
Of course, the provision of separate feed systems for each of the particulate materials greatly increases the complexity of the fluidized bed furnace system and it also adds significantly to the capital cost and the operating cost associated with the fluidized bed furnace system. Additionally, the drying of the wet coal by flash drying evaporation requires a significant amount of inert hot gas and a duct system which provides sufficient residence time for contact between the hot inert gas and the wet coal to permit evaporation. An inert gas, such as flue gas, must be used rather than fluidizing or combustion air as the drying gas must be unreactive with respect to the coal in order to prevent explosions. Additionally, cleanup equipment such as the cyclone separators and the bag filter system must be provided to remove the coal from the drying gas in order that the drying gas may be clean enough for venting to the atmosphere. The provision of the cleanup system, of course, increases the capital cost of the fluidized bed furnace system and complicates maintenance and operation of the fluidized bed furnace system.
One alternative to drying the wet coal with an inert gaseous medium is to dry the wet coal with an inert particulate medium. One such system is disclosed in U.S. Pat. No. 4,414,905 wherein hot particulate material is removed from the fluidized bed and contacted with wet coal in a fluidized mixer. Moisture is evaporated from the wet coal in the mixer upon contact with the hot particulate solids. The moisture evaporated from the coal is entrained in the fluidizing medium and vented from the mixer. Of course, a cleanup system would be necessary to remove any entrained particulate from the vented fluidizing medium unless the vented fluidizing medium was directed back to the combustor. The dried coal and cooled particulate material are then passed to a pneumatic feeder for conveyance to the fluidized bed within the furnace through a conventional underbed feed system.
Another method for drying wet coal with particulate material is disclosed in U.S. Pat. No. 4,411,879. As disclosed therein, the particulate material elutriated from the bed in the flue gas is removed from the flue gas in a filtering means. The particulate material is then passed from the filter means and mixed with the wet coal at a temperature from about 200 degrees F. to about 400 degrees F. As the particulate material elutriated from the bed and collected in the filtering means contains unreacted calcium oxide sulfur absorbant, a hydration reaction will occur between moisture from the wet coal and the calcium oxide to form calcium hydroxide and liberate heat which will further evaporate moisture from the coal. The dry coal and the particulate material utilized in the drying process are pneumatically conveyed from the mixing vessel to the fluidized bed furnace. Additional water may be added in the mixing vessel to provide sufficient water to fully hydrate the unreacted calcium oxide in the recycle material in the event there is insufficient moisture in the coal. Fresh limestone and additional coal are fed to the fluidized bed boiler through separate conventional feed systems.
It would be advantageous to provide a single transport system wherein the feeding of the various particulate materials to be supplied to the fluidized bed is integrated to provide a single unified particulate feed system. Such a unified particulate feed system would certainly have a lower capital cost than conventional separate feed system and would present a less complex operation procedure and simplify maintenance problems. Additionally, utilizing a unified particulate feed system wherein no gaseous fluid is utilized to dry the particulate fuel eliminates the need for a gaseous cleanup system which in turn simplifies the operation and reduces capital costs.